1. Field of the Invention
The invention relates generally to methods for secondary recovery of crude oil from oil strata formations, the invention particularly relating to a method and apparatus for indirectly heating recovered crude oil by means of solar energy and injecting the heated crude oil back into the oil formation in order to reduce the viscosity of oil remaining in the formation, thereby to facilitate recovery of the remaining oil.
2. Description of the Prior Art
Since primary oil recovery methods known in the art only recover a minor percentage of the crude oil existing in an oil formation, extensive attention has been given especially in the recent past to so-called secondary recovery methods intended to increase the percentage of crude oil recovered from an oil formation. Such secondary recovery methods have included the use of heating devices insertable into an oil well for heating oil in the oil-bearing strata into which the well has been drilled. Such heating devices typically comprise electrical resistance heaters which, due to the high cost of the energy required to operate said heaters, have proven to be of limited utility. Heated fluids have further been injected into an oil formation or circulated within an oil well. For example, Gilchrist, et al., in U.S. Pat. No. 3,357,487, discloses the injection of a hot aqueous driving fluid into an oil well, the heated fluid being primarily intended to increase pressure within the well rather than heat significant portions of the crude oil disposed in the oil formation. Due to the massive amounts of expensive energy required to heat any significant portion of the crude oil in an oil formation, prior attempts to recover oil by such methods have proven impractical. According to the present invention, an economically sound method for secondary recovery of crude oil from an oil formation is provided, the method including heating of the crude oil in an oil formation by pumping of a heated portion of the crude oil recovered from the formation back into the formation to reduce the viscosity of the oil remaining therein. The present method exhibits particular economical feasibility due to the effective conversion of a crude oil storage tank into a solar collector and heat exchange apparatus, which storage tank is typically located in proximity to one or more operating wells and used for temporary storage of oil pumped from the wells.